Noninvasive imaging methods that allow the tracking of critical steps in tumorigenesis and cancer progression in mouse models can impact importantly on efforts to understand tumorigenesis and to monitor the efficacy of anticancer drug candidates. Telomerase activation occurs in over 90% of human cancers as well as in mouse cancer models, and has been proposed be a diagnostic marker for cancer. Here, we propose to generate transgenic luciferase reporter mice with mouse and human genomic DNA constructs for monitoring the in vivo TERT expression, the rate-limiting step of telomerase activation. Our primary objective is to test the feasibility of using such reporters as imaging markers to monitor tumor progression in live animals. These reporters can potentially be used to image tumors in a broad spectrum of mouse cancer models and provide a standard platform for pre-clinical anticancer drug evaluation. In addition, direct comparison of mouse and human TERT promoter activities in these transgenic reporter mice may also reveal the molecular mechanisms underlying the different regulation of telomerase expression in mice and humans. Such difference may account for the discrepancies in tumor spectra between some human cancers and their mouse models.